Regeneration of cooking chemicals from spent alkaline cooking liquor

ABSTRACT

A process for recovering useful chemicals from spent alkaline cooking liquor of high sulfide content produced in the course of wood pulp manufacture. The spent cooking liquor is concentrated and the concentrate burned in a recovery furnace to produce a smelt comprising mainly Na2CO3 and Na2S. Part or all of the Na2CO3 is separated from the smelt and reacted with SO2 from the recovery furnace flue gases and, if desired, SO2 from another source to produce Na2SO3. The Na2SO3 is burned in the recovery furnace with the concentrated cooking liquor where said Na2SO3 is converted to Na2S. The resultant smelt has a high sulfide content and, when it is dissolved in water, an alkaline cooking liquor of high-sulfide content is regenerated.

United States Patent Tadashi Nakafuri Hiroshima-ken;

Saburo Fukui, Hiroshima-ken; Masao Ono, Hiroshima-ken; Kyoichi Oku,Tokyo; Hiroshi Matsuura, Tokyo; Hltoshl Sotobayashi, Tokyo, all of Japan732,657

May 28, 1968 Nov. 2, 1971 Mitsubishi Jukogyo Kabushlki Kaisha; JujoPaper Co., Ltd.

[ 72] Inventors [21 Appl. No. [22] Filed [45] Patented [73] AssigneesTokyo, Japan [32] Priority May 31, 1967 [33] Japan [3 l 42/34754 [54]REGENERATION OF COOKING CHEMICALS FROM SPENT ALKALINE COOKING LIQUORPrimary Examiner-S. Leon Bashore Assistant Examiner-Arthur L. CorbinAttorney-McGlew and Toren ABSTRACT: A process for recovering usefulchemicals from spent alkaline cooking liquor of high sulfide contentproduced in the course of wood pulp manufacture. The spent cookingliquor is concentrated and the concentrate burned in a recovery furnaceto produce a smelt comprising mainly Na,CO and Na s. Part or all of theNa,CO,, is separated from the smelt and reacted with $0, from therecovery furnace flue gases and, if desired, SO from another source toproduce Na,SO The Na SO is burned in the recovery furnace with theconcentrated cooking liquor where said Na SO: is converted to Na s. Theresultant smelt has a high sulfide content and, when it is dissolved inwater, an alkaline cooking liquor of high-sulfide content isregenerated.

.PATENTED NUVZ I97! 36 11,434

CHlPS 1 2 5 PULP FNG2SO4 5 'I FLUE GAS i l 9 6 I H20 1 l "1 l I l 12REGENERATION OF COOKING CHEMICALS FROM SPENT ALKALINE COOKING LIQUOR Thepresent invention relates to a method for recovering chemicals fromspent cooking liquor produced in the course of wood pulp manufacturingprocesses and more particularly to a novel method for recovering cookingchemicals from alkaline spent cooking liquor having a high sulfidecontent mainly consisting of Na s or Na s and at least one of the groupNaI-IS, NaOH and Na CO The chemical pulp manufacturing method may bedivided broadly into two processes: an alkaline cooking process in whichNaOH and Na s are used as the main cooking chemicals and sulfite cookingprocesses in which sulfites such as Na SO and NaI-ISO are used as themain cooking chemicals. Most typical of the alkaline cooking process isthe kraft process. The cooking chemicals used in the kraft processconsist mainly of Na S and NaOH which are present usually in amounts of20 to 30 percent and 80 to 70 percent, respective ly, and have a sulfidecontent of 20 to 30 percent.

This sulfide content is regulated by the chemical recovery methodemployed in the kraft process. Namely, in the kraft process, spentcooking liquor released from a digester is collected and concentrated,and then the organic materials dissolved out of wood during cooking andthe inorganic materials added as cooking agents are burned in a recoveryfurnace and sodium and sulfur, which were contained in the cookingchemicals, are recovered in the form of a smelt. In the smelt, sodiumand sulfur are obtained in the form of Na,co, and Na s, respectively. Nas and Na CO obtained in the form of a smelt are then dissolved in waterto form an aqueous smelt solution and then rendered alkaline to changeNa CO into NaOH to thereby regenerate liquid cooking chemicalsconsisting of Na S and NaOH usable in the kraft process. Since smallamounts of sodium and sulfur contained in the mixture are lost duringthe cooking and recovering steps, Na SO, is added in the recoveryfurnace to make up for the loss of said components. As is commonlyknown, most of the N SO is reduced in the recovery furnace to come outin the form of Na S in the smelt. The sulfide content of the kraftcooking liquor regenerated by adding such makeup chemicals as disclosedabove is, at the highest 20 to 30 percent according to the presentlyavailable methods. When a sulfide content is above that level isrequired a specific treatment is necessitated. The primary reason thatthis sulfide content is limited within said range is that although theamounts of loss of sodium as Na S and sulfur fractions during thecooking and recovering steps are substantially equal on a molar basis asis seen from the fact that the makeup chemical for such loss is N 80.the loss of sulfur is considerably larger than that of sodium. This lossof sulfur takes place either due to reaction with Na O to form compoundssuch as Na s, Na,SO or Na SO or in the form of gases not combined withNa O, such as H 8 or CH SI-I. The amount lost in these gaseous forms isseveral times as large as the amount lost due to reaction with Na O.Particularly loss of sulfur is mostly due to losses in the form of SO,gas from the recovery furnace. Therefore, to enhance the sulfide contentof the regenerated cooking chemicals, it is necessary to reduce to aminimum the loss of sulfur. Particularly, it is extremely important torecover in some way or other said gaseous sulfur fractions and to fixthem in the form of sodium sulfide.

The present invention is intended to make possible production ofhigh-sulfide content alkaline cooking liquor by recovering and fixingsaid gaseous sulfur fractions in the form of Na s and by enhancing thesulfide content of the smelt. It is also possible to enhance the sulfidecontent according to a recovery method of the conventional kraft processby sharply reducing the collecting rate of spent cooking liquor andincreasing the loss of sodium, with the consequent necessity to increasethe amount of makeup NA SO,. However, such a method is actuallyimpractical because of extraordinary high cost resulting from anincrease in the amount of the makeup chemicals.

Now, the discussion will be directed to the possible infiuence ofsulfide content in alkaline cooking processes. The only prior method foralkaline cooking had been so-called soda cooking in which NaOI-I alonewas used as the active chemical, until the kraft recovery process wasdiscovered. Nowadays, kraft cooking in which the above-said chemicals of20 to 30 percent sulfide content are used, predominates. ln alkalinecooking, as is known, lignin in wood undergoes alkaline hydrolysis andis dissolved in the cooking liquor, but when NaOH alone is present,condensation of lignin may take place. In the copresence of Na s,however, such recondensation is suppressed and elimination of ligninfrom the wood is promoted.

TABLE 1 Effect of Sulfide Content in Alkaline Cooking of PinusDensifiora Sulfide Content of cooking liquor Permanganate number (25ml.)Effective alkali charge (as Na,0) per 0.D. chip) Total yield (5)Unbleached pulp viscosity (C.P.S.)

Cooking Conditions Max temp. ('C.) Time to max. temp.

(min.) Time at max. temp. (min.) Liquor to wood ratio 4 4 4 4(l./kg-O.D. wood) I70 I70 I70 The present invention is intended toenable recovery of useful chemicals from spent liquor in high-sulfidecontent alkaline cooking. As mentioned above, according to theconventional kraft recovery method, it is impossible to regeneratecooking chemicals of high-sulfide content without increasing the amountof makeup Na SO,,, resulting in an economical disadvantage. The presentinvention oversteps the limit of sulfide content imposed intrinsicallyin the conventional kraft recovery method and provides quite a novelrecovery process which permits regeneration of reusable cookingchemicals from spent liquor produced in high-sulfide content alkalinecooking.

The present inventioncomprises thesteps of concentrating alkaline spentcooking liquor, burning same in a conventional recovery furnace,separating a part or all of the Na CO; in the resultant smelt, sulfitingthis Na,co, with 80, gas in the flue gases and, if need be, with S0 gasfrom another source, circulating the resulting Na S0 to the recoveryfurnace, burning same with spent liquor in said furnace, fixing sulfurscattered from the furnace in the smelt to obtain a high-sulfide contentsmelt, and thereby regenerating alkaline cooking liquor of high-sulfidecontent. It is possible to regenerate alkaline cooking liquor of anydesired sulfide content by changing the circulating amount of Na CO inwhich S0 gas in said flue gases is absorbed.

Sodium and sulfur added as active chemicals in alkaline cooking arepartly lost in the unbleached pulp washing step. Particularly, sulfurforms a small amount of volatile organic compounds such as CI-I SI-I andis also lost during the cooking and unbleached pulp washing steps.However, most of the sodium and sulfur are recovered in collected spentcooking liquor. When spent liquor containing organic substancesdissolved out of wood during cooking and also containing sodium andsulfur fractions derived from the cooking liquor is concentrated andburned in a recovery furnace, there is produced a smelt comprising Na sand Na,CO,, as the principal ingredients. It is found that a change inthe sulfide content of the cooking liquor in alkaline cooking produces asubsequent change in the smelt composition. But this relationship is notknown with certainty because the structural configuration of the sodiumand sulfur portions in spent liquor liquid is unclear and further, thereactions within the recovery furnace are not given any establishedelucidation except for some experimental rules about spent liquor in akraft process where the sulfide content is limited within the range of20 to 30 percent. Such a relationship is not completely known withrespect to high sulfide content alkaline spent cooking liquor.

With a view to recovering chemicals from spent cooking liquor having aparticularly high-sulfide content, the present inventors have conductedexperimental studies on the sulfide content of the smelts obtained byburning spent liquor resulting from cooking at high-sulfide content. Asa result, it was found that the higher the sulfide content of wasteliquid (expressed as mole percent of the ratio S/Na o), the greater willbe scattered sulfur fractions in gaseous form and the greater will bethe difference between sulfide content of the smelt and that ofspentliquor.

To cite an instance, it was found 40 if the sulfide content of spentliquor is 40 percent, 60 percent and 80 percent, the correspondingsulfide content of the smelt is respectively 35 percent, l percent and61 percent.

In other words, sulfur in spent liquor is scattered, by an amountcorresponding to the difference between the sulfide content of the spentliquor and that of the smelt, chiefly in the form of SO, gas into fluegases in the recovery furnace, so that smelt sulfide content is alwayssmaller than that of spent liquor.

in high sulfide content alkaline cooking, the amount of sulfur scatteredin the recovery furnace is so large that makeup of sulfur in the form ofNa,SO, on an amount sufficient to makeup for the loss of sodium is notsufficient to keep the smelt sulfide content high enough forhigh-sulfide content alkaline cooking. According to the presentinvention, a part or almost all of the Na,CO in the smelt is separatedto enhance the sulfide content of the smelt while SO, gas scattered intothe flue gases and, if need be, SO, gas from another source, is absorbedinto this Na CQ and the resulting Na SO returned to the recovery furnaceso as to balance the proportion of sodium and sulfur contents in therecovery system and to thereby obtain a smelt of high-sulfide content.

it is not absolutely necessary to use Na CO separated from said smeltfor absorption of the sulfur in flue gases. A part of the aqueous smeltsolution may be used for direct absorption of the sulfur in flue gasesto attain substantially the same effect.

Now, the process for practising the present invention will be explainedwith reference to the accompanying drawing in which a flow sheet showinga mode of practice ofthe chemical recovery method is illustrated.

in the drawing, 1 designates a high-sulfide content alkaline cookingreactor. The spent liquor released from the cooking reactor l isforwarded through a spent liquor oxidizing means 2 to a mixing apparatus3 where said liquor is mixed with Na,SO solution from a sulfitingapparatus 9. The spent liquor oxidizing means convert the sulfides-inspent liquor by means of oxidation with oxygen into stable materialssuch as thiosulfate or polythiosulfate which prove useful to retard thegeneration of volatile sulfur compounds in the ensuing evaporating andconcentrating step 4 and also to reduce corrosion of apparatus and toprevent the loss of sulfur components. The oxidizing means 2 may bearranged to come after the mixing apparatus 3. The mixed spent liquor isthen forwarded to the spent liquor concentrating means 4 which LII maypreferably comprise a combination of a spent liquor concentratingmultieffect evaporator and a cascade evaporator which are preferablyarranged so that said spent liquor is concentrated to about 50 to 55percent solids in the multieffect evaporator and then furtherconcentrated to about 65 percent solids in the cascade evaporator.

The concentrated spent liquor is then supplied to a spent liquor burningmeans, or a chemicals recovery furnace 5 where most of the sodium andsulfur contained in the spent liquor is formed into a smeltcomposed-mainly of Na s and Na CO in this case, Na SO is added in asuitable amount to the spent liquor supplied to the recovery furnace 5to makeup for the loss of said components suffered during each step.This makeup chemical is not confined to Na,SO,; it may be Na,S0;, gr thelike. The smelt from the recovery fumance 5 i s then guided into a smeltdissolving tank 6 where it is converted into I a smelt solution. Theconcentration of this aqueous smelt solution, in consideration of heateconomization in the ensuing crystallizing'step, should be arranged tobe as high as possible, preferably as close as possible to thesaturation point of the smelt solution. The smelt solution is furtheradvanced into a crystallizing means 7 where it is separated into acrystalline solid mainly composed of Na,CO, and mother liquor mainlycomposed of Na,S. Separated Na,CO, is redissolved in a Na C0 dissolvingtank 8. A part of this Na,CO solution goes to a causticizing step H,with the rest going to a sulfiting means 9. The amount of Na,CO, goingto the causticizing step is determined according to the sulfide contentof the alkaline cooking operation 1. in alkaline cooking where thedegree of sulfide content is percent, the amount of Na CO going to thecausticizing step 11 is zero. The remaining part of the Na CO is ledinto a sulfiting means 9 where it is contacted with a mixed solution ofNa CO and Nal-lSO from S0 gas absorbing means 10 and, while emitting C0gas from a reaction shown by the following formula, is converted into NaSO and returned to the mixing apparatus 3:

In the SOggas absorbing means 10, flue gas containing 1 to 0.3 vol.percent of S0 from the recovery furnace 5, and if additional S0 isrequired, S0 gas from a SO, gas generating apparatus 12, is contactedwith the solution consisting mainly of Na SO and nonreacted Na CO fromthe sulfiting means 9 to thereby allow said solution to absorb S0 gasaccording to the following formula:

Na cO +50 Na;SO +CO (2) NaSO S02 H 09 ZN aHSOs (3) The mixture is thenreturned to the sulfiting means 9 to be used for sulfiting of Na cO itshould also be noted that an aqueous solution of Na C0 may be directlycontacted with flue gas to sulfite it to thereby convert Na,co, into NaSo On the other hand, mother liquor deprived of crystals of Na CO by acrystal separator of the crystallizing apparatus 7 and mainly composedof Na s is mixed with a part of the Na CO solution directly or in tocausticizing apparatus ll and after converting Na CO in the mixture intoNaOH, the mixture is brought back to the cooking equipment 1 as thehigh-sulfide content alkaline cooking liquor.

To illustrate in a more detailed manner, Na,CO which was separated fromthe smelt solution in said crystallizing apparatus 7 and then sulfide inthe sulfiting apparatus 9 is brought back to the recovery furnace in theform of Na SO, which behaves in the same manner as Na,SO. in the furnaceand is converted mainly into Na,S obtained in the form of smelt. Byselecting the amount of Na,CO;, thus sulfited and brought back to therecovery furnace, it is possible to adjust the sulfide content of thesmelt. Thus by separating Na CQ- in an amount corresponding to saidcirculating amount, from the smelt by the crystallizing apparatus,alkaline cooking liquor of 25 to 100 percent sulfide content can beobtained. The sulfide content of the resulting alkaline cooking liquoris enhanced in proportion to the amount of Na CO to be circulated.

it should be noted that the instant invention is just described can alsobe applied to so-called cross recovery in which cooking chemicals arerecovered from both alkaline spent cooking liquor and sulfite spentcooking liquor. In this case, sulfite cooking may be employed during thecourse where a part or most of the Na cO in the smelt is converted intoNa S0; in the sulfiting means 9 and then guided into the mixingapparatus 3. Namely, as will be understood from the reaction formula(1), (2) and (3) described above, by controlling the sulfiting rate by aconventional method, it is possible to prepare neutral sulfitesemichemical cooking liquor comprising either Na SO alone or a mixtureof Na,SO and Na C0 or sulfite cooking liquor comprising one or a mixtureof Na SO NaHSO and H 80 In this case, neutral sulfite semichemical orsulfite spent cooking liquor is delivered to the mixing apparatus 3 andthereafter treated according to the same steps as described above.

It is thus possible to easily regenerate sulfite cooking liquor andalkaline cooking liquor at the same time without requiring anyadditional step.

in the following will be shown some examples of the present invention.

Example 1 Alkaline spent cooking liquor of 100 percent sulfide content(pH =l2.6, solid concentration 21.5 Wt. percent, S/Na O molar ratio inspent liquor 0.85) and an aqueous solution of Na SO (concentration 18Wt. percent) as the sodium component were mixed in the ratio of 4 :1 (NaO molar ratio) and the mixture was concentrated to 55 wt. percentsolids. This concentrated spent liquor was burned in an experimentalcombustion furnace. Resulting therefrom was a smelt having a compositioncomprising 61 Wt. percent of Na S, 31 Wt. percent of Na Co and 8 Wt.percent of Na SO and other materials.

This smelt was then dissolved in water and, after adjusting theresultant solution to contain 20 wt. percent solids, said solution wasplaced in a crystallizer where it was concentrated by evaporation at 60C. to crystallize out most of the Na CO in the form of Na CO -H O, whichwas then separated from the mother liquor by means of a centrifuge.Purity of the separated Na CO crystals was 95 percent. The separated NaCO was redissolved in water to produce a solution having wt. percentsolids, and the solution was then sent to the sulfiting step where itabsorbed S0 gas in the flue gas from the recovery furnace and Na SO TheS0 concentration in the flue gas was 0.4 vol. percent and the rate ofabsorption of 80,, gas was approximately 90 percent. The pH of theaqueous Na SO solution after absorption of $0 was about 7.8 while theconcentration thereof was 18 wt. percent.

This Na SO solution is mixed with alkaline cooking spent liquor. I

On the other hand, the concentration of Na S in the mother liquorobtained in the crystallizing step was about 20 wt. percent calculatedas Na S and about 4.6 wt. percent calculated as Na CO This can be usedas 100 percent sulfide content alkaline cooking liquor with no need forcausticizing.

Example 2 Alkaline spent cooking liquor of 100 percent sulfide contentand neutral sulfite semichemical spent liquor (pH=6.2, solidconcentration 4.8 wt. percent, S/Na,0 molar ratio in spent liquor 0.98)were mixed in the ratio of solids in the alkaline spent cooking liquor:solids in the neutral sulfite semichemical spent liquor 3:1 (by weight)and the mixture was concentrated to 55 wt. percent in terms of solids.The pH of the mixed spent liquor was 10.5 to l 1.0. As a result of thecombustion of this concentrated spent liquor, there was obtained acomposition comprising 50 wt. percent of Na s, 35 wt. percent of Na COand 15 wt. percent of Na,S0 plus other materials.

Then, in the same manner as in example 1, the smelt was dissolved inwater and Na CO was crystallized under the same conditions as in example1 and separated from the mother liquor. Thus obtained were crystallizedNa,co. of about percent purity and mother liquor containing mainly Nags.

Separated Na CO was dissolved in water and then sulfited to obtain anaqueous solution of Na,s0,. The mother liquor is used as alkalinecooking liquor while aqueous Na,SO solution is used as neutral sulfitesemichemical cooking liquor.

As explained above in detail, it is possible with the present inventionto easily recover useful chemicals from high sulfide content alkalinespent cooking liquor, thus making it possible to practice high-sulfidecontent alkaline cooking on an industrial scale. Further, in plantswhere both alkaline cooking and sulfite cooking equipments are jointlyprovided, the present invention can also be employed to recover cookingchemicals from spent liquor released in both pulping process. Thus thepresent invention is extremely beneficial in industrial applications.

What is claimed is:

1. In a process for regenerating cooking chemicals from spent alkalinecooking liquor containing Na,S and Na CO in which said spent alkalinecooking liquor is concentrated and the concentrated spent cooking liquoris burned in a recovery furnace to produce a smelt comprising Na s andNa- C0 and a flue gas containing S0 the improvement comprising:

a. dissolving the smelt from a recovery furnace to form a smelt solutionand crystallizing the smelt solution to form solid Na CO and a motherliquor mainly composed of Na S, and separating the mother liquor fromsubstantially all ofthe Na cO b. redissolving the solid Na CO in water,and contacting 50 from the flue gas therewith to convert said NzuCOsolution to a Na SO solution;

. recycling the resultant Na SO solution to fresh concentrated spentcooking liquor to form a mixed solution prior to any burning thereof insaid recovery furnace; and

d. burning the resultant mixed solution in said recovery furnace wherebyNa SO contained therein is converted to Na s to be used in cookingliquor.

2. In a process for regenerating cooking chemicals from spent alkalinecooking liquor containing Na S and Na Co in which said spent alkalinecooking liquor is concentrated and the concentrated spent cooking liquoris burned in a recovery furnace to produce a smelt comprising Na s and Nco. and a flue gas containing S0 the improvement comprising:

a. dissolving the smelt in water and contacting S0 from the flue gastherewith to convert the Na CO contained therein to Na SO to form asolution containing Na S0 and Na s;

b. recycling the resultant solution from (a) to fresh concentrated spentcooking liquor prior to the burning thereof in said recovery furnace;and

c. burning the resultant mixed solution in said recovery furnace wherebyNa so contained therein is converted to Na S to be used in cookingliquor.

3. The process of claim 1 wherein the Na,SO solution is used in asulfite cooking process prior to being added to concentrated spentcooking liquor and burned in the recovery furnace.

4. The process of claim 1 wherein the spent alkaline cooking liquorcontains in addition to Na s and Na,CO, at least one member of the groupconsisting of NaOH and NaHS.

2. In a process for regenerating cooking chemicals from spent alkalinecooking liquor containing Na2S and Na2CO3 in which said spent alkalinEcooking liquor is concentrated and the concentrated spent cooking liquoris burned in a recovery furnace to produce a smelt comprising Na2S andNa2CO3 and a flue gas containing SO2, the improvement comprising: a.dissolving the smelt in water and contacting SO2 from the flue gastherewith to convert the Na2CO3 contained therein to Na2SO3 to form asolution containing Na2SO3 and Na2S; b. recycling the resultant solutionfrom (a) to fresh concentrated spent cooking liquor prior to the burningthereof in said recovery furnace; and c. burning the resultant mixedsolution in said recovery furnace whereby Na2SO3 contained therein isconverted to Na2S to be used in cooking liquor.
 3. The process of claim1 wherein the Na2SO3 solution is used in a sulfite cooking process priorto being added to concentrated spent cooking liquor and burned in therecovery furnace.
 4. The process of claim 1 wherein the spent alkalinecooking liquor contains in addition to Na2S and Na2CO3 at least onemember of the group consisting of NaOH and NaHS.